1. Field of the Invention
The present invention relates to an improvement in a heat-insulation type exhaust manifold for an engine, which includes an upper flange, a plurality of exhaust single pipes coupled at their upstream ends to the upper flange, an exhaust collecting pipe coupled to the downstream ends of the exhaust single pipes, and a lower flange coupled to the downstream end of the exhaust collecting pipe, each of the pipes being formed in a double-wall fashion.
2. Description of the Prior Art
There is a conventionally known heat-insulation type exhaust manifold, for example, as disclosed in Japanese Patent Application Laid-open No. 9-280046, which is comprised of an upper flange, a plurality of exhaust single pipes coupled at their upstream ends to the upper flange, an exhaust collecting pipe coupled to the downstream ends of the exhaust single pipes and communicating with the exhaust single pipes, a common outer pipe which covers the plurality of exhaust single pipes and the exhaust collecting pipe and which is coupled at its upstream end to the upper flange, and a lower flange coupled to the downstream end of the outer pipe. The downstream end of the exhaust gas collecting pipe is slidably carried on an inner peripheral surface of the outer pipe. The difference in axial thermal elongation between the exhaust single pipes as well as the exhaust collecting pipe and the outer pipe is absorbed by the sliding movement between the exhaust gas collecting pipe and the outer pipe, so that a thermal strain is prevented from being produced in each of respective portions to the utmost.
In the above known exhaust manifold, however, the plurality of exhaust single pipes are integrally coupled to the exhaust collecting pipe. For this reason, all the thermal elongation of each of the exhaust single pipes is concentrated in the sliding portions of the exhaust collecting pipe and the outer pipe, and a large sliding stroke must be ensured in the sliding portions. However, if the sliding stroke is increased, there is a possibility that an inclination occurs between the sliding portions in the sliding course and as a result, a thermal strain is liable to be produced.